Because of its being thin, light in weight, and drivable by a low voltage, an LCD device is extensively employed in various electronic devices.
In a typical LCD device, a backlight module having a light source and a light guiding device is used to provide the needed illumination. The light source emits light beams into the light guiding device, which then transmits the light beams to illuminate liquid crystal molecules in a liquid crystal panel. It is important that the light guiding device transmits the light beams to the liquid crystal panel uniformly.
Referring to FIG. 3, this is a schematic, side view of a typical backlight module 2. The backlight module 2 includes a light guiding plate 21 and a linear light source 20. The light guiding plate 21 includes an incident surface 211, a light emitting surface 213 adjacent to the light incident surface 211, and a sawtooth bottom reflective surface 212 opposite to the light emitting surface 213. The light source 20 is arranged in the vicinity of the incident surface 211.
FIG. 4 is an enlarged view of part of the backlight module of FIG. 3, showing essential optical paths thereof. AB1, AB2, and AB3 represent light beams emitted by the light source 20 that enter the light guide plate 21, and are reflected by the protrusions 214 of the reflective surface 212. The light beams AB1, AB2, and AB3 then transmit to the light emitting surface 213, and emit out of the light guiding plate 21 for illuminating an LCD device.
The protrusions 214 of the reflective surface 212 have a regular configuration. When the light beam AB3 is reflected by one of the protrusions 214 that is far away from the light source 20, the light beam AB3 has a larger incident angle than that of the light beams AB1 or AB2 that are reflected by protrusions 214 nearer to the light source 20. This means that the light beam AB3 is liable to be totally internally reflected within the light guiding plate 21. Thus, at least some of the energy of the light beam AB3 is apt to be lost. Effective utilization of the light beams of the light source 20 may be less than satisfactory.
What is needed, therefore, is a new light guiding plate and a corresponding backlight module that overcome the above-described problems.